The capacity to catalyze recombination at designated genomic sites in the mouse germline would provide the means for engineering specific rearrangements. Specifically, deletions and duplications of defined regions and integration at specific sites will answer a variety of questions. Among these specific questions are the mapping of the genes responsible for the pleiotropic phenotypes of Down's Syndrome, recapitulated in a murine segmental trisomy and the deletion mapping of promoter elements at the enkephalin locus. Recently, the bacteriophage P1 Cre recombinase has been shown to be active in catalyzing recombination between lox sites in mammalian cells, including pluripotent embryonic stem cells and mouse oocytes. We are using this system, first to engineer specific segmental duplications on mouse chromosome 16 and, second, to introduce specific promoter deletions of the enkephalin gene at the HPRT locus. In the first part of the project, lox sites will be introduced by homologous recombination at specific sites on chromosome 16 in embryonic stem cells. Recombination between these sites will then be catalyzed by the Cre recombinase in embryonic stem cells and in oocytes. In the second part of the project, a lox site has been introduced at the HPRT. Further, promoter deletions of the enkephelin locus have been constructed in plasmid sectors with lox sites. The plasmids will be integrated at the HPRT site by cotransfection into the embryonic stem cell lines with a Cre expressing plasmid. While these experiments do not exhaust the possibilities of this approach, they will demonstrate its effectiveness for further applications.